HMGB1 (high mobility group box chromosomal protein 1), historically known as an abundant, nonhistone architectural chromosomal protein, is extremely conserved across species. As a nuclear protein, HMGB1 stabilizes nucleosomes and allows bending of DNA that facilitates gene transcription. Unexpectedly, recent studies identified extracellular HMGB1 as a potent macrophage-activating factor, signaling via the receptor for advanced glycation end-products to induce inflammatory responses. It is released as a late mediator during inflammation and participates in the pathogenesis of systemic inflammation after the early mediator response has resolved. HMGB1 occupies a critical role as a proinflammatory mediator passively released by necrotic but not apoptotic cells. Necrotic Hmgb1(-/-) cells mediate minimal inflammatory responses. Stimulated macrophages actively secrete HMGB1 to promote inflammation and in turn, stimulate production of multiple, proinflammatory cytokines. HMGB1 mediates endotoxin lethality, acute lung injury, arthritis induction, activation of macrophages, smooth muscle cell chemotaxis, and epithelial cell barrier dysfunction. HMGB1 is structurally composed of three different domains: two homologous DNA-binding sequences entitled box A and box B and a highly, negatively charged C terminus. The B box domain contains the proinflammatory cytokine functionality of the molecule, whereas the A box region has an antagonistic, anti-inflammatory effect with therapeutic potential. Administration of highly purified, recombinant A box protein or neutralizing antibodies against HMGB1 rescued mice from lethal sepsis, even when initial treatment was delayed for 24 h after the onset of infection, establishing a clinically relevant therapeutic window that is significantly wider than for other known cytokines.